The security guard keep their eyes on these guys (8)

I believe the answer is:

watchmen

I'm a little stuck... Click here to teach me more about this clue!

'the security guard keep their eyes' is the definition.
I can't tell whether this definition defines the answer.

'on these guys' is the wordplay.
'on' is a charade indicator (letters next to each other).
'these' becomes 'watch' (I can't justify this - if you can you should believe this answer much more).
'guys' becomes 'men' (guy can mean a man).
'watch' next to 'men' is 'WATCHMEN'.

Can you help me to learn more?

(Another definition for watchmen that I've seen is " Sentinels".)

I've seen this clue in the Sydney Morning Herald.

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